1. Technical Field
This invention relates to a wrap spring clutch and more particularly to an electromagnetically actuated wrap spring clutch designed to reduce assembly time, cost, and clutch engagement time.
2. Discussion of the Background Art
A conventional, electromagnetically actuated, wrap spring clutch includes an output shaft, a shaft hub disposed about the shaft and connected for rotation therewith, and an input hub. The input hub is also disposed about the shaft, but may rotate independently of the shaft when the wrap spring clutch is de-energized. The clutch further includes a coil substantially disposed about the shaft hub and an annular wrap spring disposed about a portion of the input hub and a portion of the shaft hub. A first end of the wrap spring is connected to the input hub so that the spring rotates with the input hub. A second end of the wrap spring is disposed radially outwardly of the shaft hub and is free of the shaft hub when the clutch is de-energized. Energizing the coil establishes magnetic flux circuits or closed loops in the magnetically permeable portions of the clutch. Attractive forces arising from the flux draw a second end of the spring into contact with the shaft hub (which may be non-rotating at this point). Frictional forces restrain the second end of the spring from rotating. The difference in relative rotation between the first and second ends of the spring causes the spring to wrap down upon the shaft hub, thereby transmitting torque from the input hub to the shaft hub and output shaft.
The wrap spring used in a conventional spring clutch includes a first portion having a first diameter and a second portion having a second diameter different from the first diameter. The portion having the larger diameter is disposed about the shaft hub to allow clearance between the wrap spring and the shaft hub when the clutch is de-energized. The portion having the smaller diameter is disposed about the input hub and a plurality of turns of the wrap spring interfere with the input hub to secure the spring and minimize engagement time for the clutch. The use of a multi-diameter wrap spring, however, increases the manufacturing costs of the clutch and the interference between the spring and the input hub makes it difficult to assemble the wrap spring onto the input hub.
The wrap spring used in a conventional spring clutch also includes first and second tangs at either end of the spring. The first tang extends radially inwardly toward the center of the coil formed by the spring and secures the spring to the input hub. The second tang extends radially outwardly and is connected to a control collar disposed outwardly of the shaft hub. The use of radially inwardly and radially outwardly extending tangs, however, has several disadvantages. First, when assembling the wrap spring to the input hub, the wrap spring must be opened sufficiently to position the inward tang over the diameter of the input hub. This can result in damage to the wrap spring and/or damage to the input hub if the tang scratches the hub surface. Second, the spring must be properly oriented during assembly thereby increasing assembly time.
There is, therefore, a need for an electromagnetically actuated wrap spring clutch that minimizes or eliminates one or more of the above-mentioned problems.
The present invention provides an electromagnetically actuated wrap spring clutch.
An electromagnetically actuated wrap spring clutch in accordance with the present invention includes a shaft hub disposed about a longitudinal axis and an input hub that is coaxial with the shaft hub. The input hub includes a first portion having a first diameter and a second portion having a second diameter wherein the second diameter is smaller than the first diameter and the second portion is nearer the shaft hub than the first portion. The clutch also includes a spring disposed radially outwardly of a portion of the shaft hub and the first and second portions of the input hub and connected to the input hub. The spring may be symmetrical, having a uniform diameter and including first and second tangs disposed at either end of the spring wherein both tangs extend radially outwardly. The clutch further includes a control collar connected to the spring and disposed radially outwardly of the spring and a coil disposed about the shaft hub.
A wrap spring clutch in accordance with the present invention has several significant advantages over conventional wrap spring clutches. First, the inventive clutch minimizes clutch engagement time because, given the equal number of wraps, the spring is able to wrap down onto the shaft hub more quickly. Second, the inventive clutch requires less assembly time because: (i) the reduced diameter portion of the input hub allows the wrap spring to be more easily assembled onto the input hub; (ii) the symmetrical spring makes orientation of the spring irrelevant during assembly; and (iii) both tangs of the spring extending outwardly thereby eliminating the need to expand the spring prior to installation on the input hub. Third, the inventive clutch is less likely to be damaged during assembly because both tangs of the spring extend radially outwardly, thereby eliminating any contact between the tangs and the input hub surface. Finally, the inventive clutch costs less than conventional clutches due to the use of a less costly symmetrical spring and the reduction in assembly time.